The invention relates to a hydrodynamic type oil-impregnated sintered bearing in which its porous bearing body of sintered metal is impregnated with lubricating oil or lubricating grease as a lubricant so as to have a self-lubricating function, and the hydrodynamic pressure effect of hydrodynamic pressure generating grooves in the bearing surfaces thereof forms a lubricating film in the bearing clearance, so that the lubricating film non-contact supports the sliding surfaces of a rotating shaft. The hydrodynamic type oil-impregnated sintered bearing according to the present invention is suitably applied to spindle motors for information equipment. Of these, especially suitable applications are those required for high rotational accuracies at higher speeds, such as a polygon scanner motor in a laser beam printer (LBP) and a spindle motor for a hard disk drive (HDD), and those driven at higher speeds under a condition in which the loading of disks produces an unbalanced load, such as a spindle motor for optical disk devices including DVD-ROMs and DVD-RAMS, and for magneto-optical disk devices including MOs.
Information equipment is generally divided into two types: main storages for data processing and storing, and secondary storages for storing only. Storing components thereof are classified into those using disks or tapes, and those consisting of electronic parts only. At present, disks and tapes are widely used in view of cost. The secondary storages using disks and tapes include magnetic disk drives (HDD, FDD), optical disk drives (CD, DVD), magneto-optical disk drives (MO, ODD), and digital audio tape recorders (DAT). Information equipment further includes laser beam printers (LBP), digital FAXs, and digital PPCs.
Such small-size spindle motors for information equipment as described above are required for further-improved rotation performance, lowered noises, and lowered cost. For those means, studies have been made on the replacement of rolling bearings for the spindles with oil-impregnated sintered bearings. However, since an ordinary oil-impregnated sintered bearing is a sort of cylindrical bearing, unstable vibration easily occur in its shaft at smaller eccentricities. This leads to a drawback of the easy occurrence of a so-called whirl, in which the shaft deviates around at a speed ½ the rotating speed. (The occurrence of unstable vibrations such as a whirl deteriorates the rotational accuracy.) Approaches thereto have been previously made in which hydrodynamic pressure generating grooves of herringbone type, spiral type, and the like are provided in the bearing surfaces so that the hydrodynamic pressure effect of the hydrodynamic pressure generating grooves created with the rotation of the shaft enhances the bearing functions such as radial rigidity for suppressing the shaft run-out caused by the unstable vibrations (hydrodynamic type oil-impregnated sintered bearings).
A hydrodynamic type oil-impregnated sintered bearing is characterized in that the oil retained in pores inside the bearing body forms a lubricating film in the bearing clearance by the effect (the oil-drawing effect) of the hydrodynamic pressure generating grooves while circulating between the bearing body and the bearing clearance, so that the lubricating film continuously non-contact supports a rotating shaft. In order to develop such a stable bearing function, proper circulation of the oil and secured formation of the lubricating film needed for supporting the shaft are required. One of the essential factors thereto is the selection of a lubricant to be impregnated into the bearing body.
An ordinary cylindrical bearing (oil-impregnated sintered bearing having no hydrodynamic pressure generating groove in the bearing surfaces thereof) uses, for example, lubricating oil of poly-α-olefin compounded with various types of additives, as disclosed in Japanese Patent Laid-Open Publication No. Hei 7-53984. The lubricating oil has excellent properties for lubricating oils dedicated to oil-impregnated sintered bearings. The properties include small generation of sludge in use, a wide available temperature range, excellent lubricity, high conformability at lower torques, and good durability. However, it has been found that the using of this oil as the impregnation oil for a hydrodynamic type oil-impregnated sintered bearing sometimes produces a slight whirl. No clear explanation thereto has been presently given; however, it seems to have a connection with the poly-α-olefin's tendency of producing bubbles in oil when used as impregnation oil, and the peculiar oil-drawing effect of hydrodynamic type oil-impregnated sintered bearings.
The occurrence of a whirl becomes critical especially in the cases of polygon scanner motors in laser beam printers (LBP), in which the motors are driven at high speeds of several tens of thousands of rotations, and in the applications required for non repeatable run out (NRRO), such as a hard disk drive motor (HDD), a high capacity floppy disk drive motor (Zip, HiFD), and an optical disk motor (DVD-RAM); and it fatally hampers the maintenance of accuracies such as required jitter (jitter means unstable fluctuations in pulse amplitudes and on-a-time-axis parameters of a pulsed sequence of reflected light coming from a polygon mirror, or the value of the fluctuation), NRRO, and surface run-out. Since required for lower torques as well as higher rotatabilities, spindle motors of this type use low-viscositied impregnation oil. However, poly-α-olefin is high in evaporation, and even higher at lower viscosities; therefore, need for a long endurance life is not always satisfied at higher speeds and in high-temperatured atmospheres.
Besides, although the hydrodynamic type oil-impregnated sintered bearings of this type have an high effect of suppressing run-out, they also have a phenomenon of lowering the hydrodynamic pressure effect (pressure drop) due to the run-off of the oil from the bearing clearance to the inside of the bearing body through surface holes in the bearing surfaces, which gives rise to a problem in that the expected hydrodynamic pressure effect is hard to obtain. As means for solving the pressure-drop problem, a constitution has been conventionally known in which a surface filling-up process is applied to the hydrodynamic pressure generating grooves in the bearing surfaces to seal the forming areas of the hydrodynamic pressure generating grooves (Japanese Patent Laid-Open Publication No. Sho 63-19627).
However, the constitution with the sealed forming areas of the hydrodynamic pressure generating grooves leads to problems as follows:    1. Since the forming areas of the hydrodynamic pressure generating grooves are completely sealed, the oil circulation, which is the greatest feature of oil-impregnated sintered bearings, is hampered in the areas. Accordingly, once exuded into the bearing clearance, the oil is driven to axial central portions on the bearing surfaces by the action of the hydrodynamic pressure generating grooves, and left in the bearing clearance. In the bearing clearance, large shearing action is at work; hence, the left oil is easily denatured by shearing forces and frictional heats there, and accelerated in degree of oxidation degradation due to an increase in temperature. This consequently shortens the life of the bearing.    2. The proposed means for the surface filling-up include coating and the like. However, coating films in the coating need to be thinner than the depth of the grooves, and it is extremely difficult to apply such thin coating films of several micrometers onto only the forming areas of the hydrodynamic pressure generating grooves.
Besides, Japanese Patent Laid-Open Publications No. Sho 63-195416 and No. Hei 7-42740 describe techniques of impregnating a sintered article with lubricating grease (oil-impregnated sintered bearings); however, these techniques are intended for cylindrical bearings having no hydrodynamic pressure generating groove in the bearing surfaces, and offer so small radial rigidities in a domain of small eccentricities that the unstable vibrations such as a whirl cannot be suppressed effectively.